GPS mis-placed Triple-T

<div class='quotetop'>QUOTE(yoda\";p=\"98833)</div>

I don't have Prius (yet), but have used handheld GPS units for years (the reason being www.geocaching.com) and cloud and tree cover, tree lines, tunnels, urban canyons, etc effect GPS accuracy, 75 feet off on a very cloudy day would not surprise me.

I think I read someplace about the Prius / Lexus nav sensing when things were awry with the GPS signals and it used car speed/direction to compensate, which might have happened in your case.

 

You probably lost site of the satellites and the NAV's dead reckoning system took over to "guestimate" where you were and then self-corrected when it could see the sats again.

 

The GPS receiver in the Prius (or any car for that matter) likely has a horizontal accuracy of about 15 meters. It would be very costly to equip a vehicle with a more accurate (submeter) receiver.

A GPS receiver determines location by measuring the signal travel time from the satellite to the receiver, assuming that the signal travels at the speed of light. Atmospheric and multipath errors (mentioned above) can increase the travel time. Multipath errors, for example, occur when the signal is reflected off of another surface, such as a building or a body of water.

Another term you might hear is Position Dilution of Precision (PDOP), which refers to satellite geometry. Essentially, the PDOP increases when satellites are closer together, resulting in an increased potential for error. If the satellites in view are farther apart, PDOP usually goes down (with a decreased potential for error).

Errors can also be introduced with the Ephemeris data, as well as what is called clock drift. Ephemeris data is transmitted from the satellite to the receiver, and contains information about satellite position. If this information is in error, then the receiver will think the satellite is in the wrong location. Clock drift occurs when the atomic clock on the satellite is "off". For example, a one nanosecond discrepancy can translate into a one foot error on the ground.

Any of these errors could account for the 75 foot (or more) offset that you experienced. Since the database doesn't know any better, it just snaps you to the closest street.

 

Last fall the NAV told me that I was about 1/2 mile to the left. But 1/2 mile to the left was out over the bay. As the icon approached land, the icon kept jumping to nearby streets. When the street on the display ended, the icon seemed to stop and then jump to another street.

This happened for about 60 seconds and then bingo, the icon jumped to where we actually were.

Another time, the icon seemed to be about 100 feet off.

This is now twice in 12000 miles. The first time was during a thunderstorm, the second time was a normal sunny day.

Other than those two times, I've been very impressed with the accuracy of the NAV.

I love this car.

 

[font=Comic Sans MS:d0d2745cee]Yoda, when I start out in the morning it frequently takes several minutes for the nav to sense the GPS signals. Take a look at the map display and if you don't see the letters "GPS" under the scale icon, you are using dead reckoning.

Dead reckoning works best when the inertial sensors can provide input to the nav system, to match turns to the roads in the database. It's a pretty straight shot south out of my place, and sometimes the display shows me skidding down the road sideways, until I make a few turns or GPS is acquired.

As the manual states, the nav is not a substitute for your own good sense. Don't try to drive in the bay with Prius04... [/font:d0d2745cee]

 

very curious to hear from people who have the Prius Nav. what does the Nav do with streets that are new or have been realigned?

does it still snap to the street as defined on the map or does it ignore the street data it has and give you the real position?

i ask because i use Route 66 software with a portable GPS receiver and when i went on a road that had been straightened, it showed me on my real path even when there was no road on the map.

your statement about "snapping to the nearest road" makes me wonder how the system would react to new roads that are not listed in the mapping software loaded on the DVD

 

[font=Comic Sans MS:d29bcf7a57]Hi Dave. The nav system does try to find a road if there's one nearby in the database, but if it can't, it just shows you maneuvering in space. Compass mode always shows your GPS longitude and latitude.

A new freeway exit was constructed on the way to the dealer I bought the Silver Cloud from. If I follow nav directions it will lead me astray and then back to the dealer, but it's not direct. Some people near me don't have guidance to their homes, since they are new. So nav gets you close, but you are on your own if reality doesn't match the database. [/font:d29bcf7a57]

 

I would have to agree with Bill. The software likely has a snapping tolerance (e.g. snap to the closest road if it's within 100 feet). I have no idea what the actual setting is though.

Just a note that cloud cover will not cause the GPS to lose a signal. It can affect accuracy though. Obstructions, such as tall buildings and tree foliage, will cause signal loss. Fortunately though most Prius owners likely don't drive through dense forests. For GPS field work under forested conditions it's better to collect data during leaf-off (for you southern folks, that would be in the winter or early spring for us northern folks).

The PDOP I mentioned above may cause signal loss as well. Usually GPS receivers (at least high grade receivers) have a PDOP threshold setting (e.g. don't collect data if the PDOP goes above 5). So in this case if the satellite geometry is poor, the GPS receiver won't give you a position fix.

 

It's also possible to adjust the vehicle position in the setup.
And there's a turn sensor in the car because GPS just isn't accurate enough to get heading data as quickly as you make a turn or freeway ramp.
I either fumbled the position adjustment or my GPS had a little brain-fade because it recently decided my car was about half a mile away from its actual position -- I've had to adjust to back several time now to get it exactly where it should be.

 

The "lack of accuracy" has been attributed by some here as due to using dead-reckonig instead of GPS.

In fact, most likely it's the other way around-- "GPS only" was innacurate, and until the additional dead-reckoning inputs (snapping to intersections after turns, etc) were incorprated into the solution, the 75 ft inaccuracy of GPS was evident.

 

hmmm makes me wonder what nav system i have. mine never snaps to street since it uses a portable GPS that is not specifically tied to the mapping system. so in places where i am on new streets, it just has me moving around on the map in open spaces

the mapping system is able to display the speed and direction without any delay or problems and it has yet to be inaccurate in 2D space (i have some questions about the displayed elevations but as of yet have not tried to verify the readings with any other sources)

with what some of you are saying, im not sure that i would prefer the system you have

 

<div class='quotetop'>QUOTE(DaveinOlyWA\";p=\"99232)</div>

[font=Comic Sans MS:c88e978514] To each his own, Dave. I specifically got the BC (#6) option package for the navigation and bluetooth. I love it.[/font:c88e978514]

 

dont get me wrong, built in convenience is great too.

 

Also, the Prius can adjust its dead reconing distance calculation based on what it thinks it travelled, and what GPS said it travelled. Like our tank MPGs and distances, short GPS calculations aren't accurate, but if say you drove 1000 miles east by GPS measurements, but information from the wheels says you drive 999 miles, the NAV will recalculate and increase distance per wheel turn. Sure, the GPS may be off a few feet, but that large distance can recalibrate your built-in yardstick: your wheels.

Also, as stated, slight errors in x-y positioning can be corrected when the map tells the NAV there's no more road ahead where it thinks you are, but an adjacent road continues. It then determines that you must be on the adjacent road. Also when you make perpendicular turns, if NAV is short of, or overshoots the corner, it will readjust its coordinates to the point on the map it senses you made a turn.
The difficulty can come when you turn into a driveway near the corner of the street. NAV has my house in the middle of a street, as I am the 2nd house from the corner, about 100 feet away.

 

well lets face it. pinpoint accuracy isnt needed. if you are within 100 feet of where you need to be going and cant find your way you probably shouldnt be driving